Memory system having a plurality of types of memory chips and a memory controller for controlling the memory chips

ABSTRACT

A memory controller converts controller output signals output from a controller into memory input signals according to the operation specifications of memory chips to operate, and outputs the resultant to the memory chips through a common bus. The memory controller also receives memory output signals output from the memory chips through the common bus, and converts the received signals into controller input signals receivable to the controller. This allows the single memory controller to access the plurality of types of memory chips. As a result, the memory controller can be reduced in chip size, lowering the cost of the memory system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional application, which claims the benefit of pendingU.S. patent application Ser. No. 11/505,835, filed Aug. 18, 2006 whichin turn is a divisional application of, Ser. No. 10/687,591, filed Oct.20, 2003, which in turn is a divisional application of U.S. patentapplication Ser. No. 10/057,989, filed Jan. 29, 2002, now U.S. Pat. No.6,650,593 B2, which claims the benefit of Japanese Patent ApplicationNo. 2001052484, filed Feb. 27, 2001. The disclosures of the priorapplications are hereby incorporated herein in their entirety byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a memory system having a plurality oftypes of memory chips and a memory controller for controlling thesememory chips.

2. Description of the Related Art

With the progression of semiconductor manufacturing technology andsemiconductor design technology, it has become possible to implement onewhole system on a single semiconductor chip. A semiconductor thatoperates as a single system is generally referred to as a system LSI. Asystem LSI contains, for example, an MPU core for controlling the entiresystem, peripheral cores (IP cores) having a predetermined function, anda memory core. The memory core stores programs necessary for theoperation of the system, data for the system to handle, and so on.

Recently, there have been developed portable apparatuses that handlelarge amounts of data such as moving images. When these portableapparatuses use memory capacities beyond those of the memory coresmounted on their system LSIs, it is usual to constitute the systems withsemiconductor memories (memory chips) externally attached to the systemLSIs. The reason for this is that if high capacity memory cores areincorporated into the system LSIs, the system LSIs increase in chip sizeand might drop in yield.

Furthermore, logic products such as an MPU and memory products such as aDRAM are optimized in design for respective features, and manufacturedunder respective optimum conditions. Accordingly, designing andmanufacturing the memory chips aside from the system LSIs (logic chips)can improve system performance.

FIG. 1 shows an example of the system (memory system) in which aplurality of types of memory chips are externally attached to a systemLSI. Here, a memory system refers to a set of functions of a systemconstituting the above-mentioned portable apparatus or the like that arenecessary for memory operation.

The memory system comprises a system LSI 2 and a plurality of types ofmemory chips 3 a, 3 b, and 3 c to be mounted on a printed-circuit board1. The system LSI 2 has an MPU 4 for controlling the entire system,peripheral cores (IP) 5 a and 5 b having a predetermined function, andmemory controllers 6 a, 6 b, and 6 c corresponding to the memory chips 3a, 3 b, and 3 c, respectively. The memory chips 3 a, 3 b, and 3 c arerespectively connected to the memory controllers 6 a, 6 b, and 6 cthrough buses 7 a, 7 b, and 7 c which are laid on the printed-circuitboard 1.

Conventionally, in the case of constructing the memory system from thesystem LSI 2 and the plurality of types of memory chips 3 a, 3 b, and 3c, it has been required, as described above, that the memory chips 3 a,3 b, and 3 c be individually provided with the memory controllers 6 a, 6b, and 6 c. For example, SDRAMs and flash memories have differentcommand systems and operation timing for performing write operations andread operations. Therefore, SDRAMs and flash memories have necessitatedtheir respective memory controllers when externally attached to a systemLSI. As a result, there has been a problem that the system LSI 2 growsin chip size and increases in chip cost.

Since the terminals of the memory chips 3 a, 3 b, and 3 c are connectedto the terminals of the system LSI 2 through the buses 7 a, 7 b, and 7c, respectively, the number of terminals of the system LSI 2 becomesenormous. Consequently, the system LSI 2 might be greater in chip sizedepending on the number of terminals. In worst cases, it has beennecessary to develop a new package for the number of terminals of thesystem LSI 2.

Since the plurality of memory controllers 6 a, 6 b, and 6 c are mountedon the system LSI 2, the system LSI 2 has been greater in circuit scale,requiring an enormous amount of time for design verification.

The formation of the buses 7 a, 7 b, and 7 c necessitates large numbersof wires on the printed-circuit board 1. Consequently, there has been aproblem that the wiring layers of the printed-circuit board 1 grows innumber, increasing the design cost and manufacturing cost of theprinted-circuit board 1.

Clock synchronous SDRAMs have been developed to improve the datatransmission rates of DRAMs. For other clock asynchronous semiconductormemories (including nonvolatile memories), products of clock synchronoustype are also likely to be developed.

SUMMARY OF THE INVENTION

It is an object of the present invention to reduce the costs of a memorysystem that has a plurality of types of memory chips and a memorycontroller for controlling these memory chips.

Another object of the present invention is to provide a common interfacein a memory system comprising a system LSI with a plurality of types ofmemory chips externally attached, the common interface connecting thememory chips and the system LSI for controlling the memory chips.

Still another object of the present invention is to attach clocksynchronous nonvolatile memories externally to a system LSI withfacility and lower costs.

According to one of the aspects of the memory system of the presentinvention, the memory system comprises: a plurality of types of memorychips operating in synchronization with a clock signal; a controller forissuing access requests to the memory chips; a memory controller forcontrolling the memory chips; and a common bus for connecting the memorychips and the memory controller to transmit memory input signals andmemory output signals. The memory chips include, for example, a volatilememory such as a synchronous DRAM and a nonvolatile memory such as aclock synchronous NAND type flash memory.

The memory controller converts, according to operation specifications ofthe memory chips to operate, controller output signals which thecontroller outputs to the memory controller when operating memory chips,into the memory input signals receivable to the memory chips. The memorychips receive the memory input signals and perform a read operation, awrite operation, or the like. Among the controller output signals andthe memory input signals are address signals, command signals, and writedata signals.

The memory chips output read data signals obtained through their readoperations to the common bus as the memory output signals. The memorycontroller receives the memory output signals through the common bus,and converts the received signals into read data signals (controllerinput signals) receivable to the controller. Then, the controllerreceives the controller input signals, thereby completing the readoperations of the memory system.

As described above, the memory controller converts controller outputsignals into memory input signals receivable to the individual memorychips. This allows the single memory controller to access the pluralityof types of memory chips. As a result, the plurality of memory chips canbe connected to the memory controller through the common bus, which canminimize a number of signal lines. In addition, the memory controllercan be reduced in circuit scale. The memory controller need not bedesigned anew upon each development of memory chips as heretofore.

According to another aspect of the memory system of the presentinvention, the memory output signals and the memory input signalsreceived respectively by the memory controller and the memory chipsthrough the common bus have the same input timing specificationirrespective of which of the memory chips is to operate. Similarly, thememory input signals and the memory output signals output respectivelyfrom the memory controller and the memory chips through the common bushave the same output timing specification irrespective of which of thememory chips is to operate. On this account, the memory controller canreliably access the plurality of types of memory chips having differentoperation specifications by simply adjusting the output order of thememory input signals and the acceptance order of the memory outputsignals according to the command specifications and the like of thememory chips.

For example, the input timing specification is defined by a setup timetIS and a hold time tIH with respect to an edge of the clock signal.Similarly, the output timing specification is defined by a setup timetOS and a hold time tOH with respect to an edge of the clock signal.When the setup time tOS and the hold time tOH are set longer than thesetup time tIS and the hold time tIH, the memory controller and theindividual memory chips can surely receive the memory output signals andthe memory input signals, respectively.

According to another aspect of the memory system of the presentinvention, the memory controller includes an operation memory unit, aninput/output controlling unit, and a conversion control unit. Theoperation memory unit stores the operation specifications of therespective memory chips. The conversion control unit operates theinput/output controlling unit in accordance with information from theoperation memory unit. For example, the conversion control unit has onlyto control the operation timing and the input/output direction of theinput/output controlling unit in accordance with the information fromthe operation memory unit. The input/output controlling unit operatesunder instructions from the conversion control unit, to input thecontroller output signals from the controller and output the controllerinput signals to the controller, and to output the memory input signalsto the memory chips and input the memory output signals from the memorychips. Operating the input/output controlling unit, or the interfacewith the memory chips, according to the operation specifications of therespective memory chips makes it possible to operate the memory chipsreliably without using complicated control circuits.

According to another aspect of the memory system of the presentinvention, the memory controller includes a signal holding unit. Thesignal holding unit temporarily holds the controller output signals andthe memory output signals received by the input/output controlling unit.For example, when the memory chip to be accessed is a synchronous DRAMof address multiplex system, an address signal (controller outputsignal) held in the signal holding unit is divided under the instructionfrom the conversion control unit and output in succession as a rowaddress signal and a column address signal. Similarly, when the memorychip to be accessed is a clock synchronous NAND type flash memory, astart address (controller output signal) held in the signal holding unitis divided into a plurality of packets under the instruction from theconversion control unit for successive outputs. That is, signals can beoutput to the memory chips according to the operation specifications ofthe respective memory chips.

According to another aspect of the memory system of the presentinvention, if one of the memory chips is in operation when the memorycontroller receives the controller output signal for operating anotherof the memory chips, the signal holding unit temporarily holds thiscontroller output signal. That is, the controller output signal outputfrom the controller can be held until the common bus becomes available.Since the controller output signal is held by the signal holding unit ofthe memory controller, the controller can access other devices such as aperipheral circuit, or peripheral cores, independent of the operationwait for the another memory chip. Since the controller is prevented fromexecuting useless cycles, the entire system improves in operatingefficiency.

According to another aspect of the memory system of the presentinvention, the memory controller includes an arbiter. The arbiteradjusts the order of accesses to the memory chips depending on theoperation states of the memory chips and the holding order of thecontroller output signals corresponding to a plurality of memory chipsthat are held in the signal holding unit. The arbiter is composed of forexample, programmable logics capable of reconstructing their respectivecircuit functions.

If a memory chip is using the common bus when the controller issues anaccess request to another memory chip, the arbiter keeps the access tothe another memory chip waiting until the common bus becomes available.The output controller signal output from the controller to access theanother memory chip is temporarily held in the signal holding unit.

In some cases where the controller issues access requests to a pluralityof memory chips for read operations, one of the memory chips cancomplete its read operation within the period from the start of theoperation of another memory chip to the output of a read data signal. Insuch cases, the arbiter operates the one memory chip by utilizing thevacancy of the common bus during the operation period of the another.

By dint of the arbiter, the single memory controller can operate theplurality of types of memory chips with efficiency. As a result, thememory system can be improved in data transmission rate.

According to another aspect of the memory system of the presentinvention, the memory controller and the controller are mounted on anidentical chip, being formed into a system LSI, for example. The memorycontroller itself can handle the plurality of types of memory chips, bywhich reduces the circuit scale. As a result, the system LSI where thememory controller is mounted can be reduced in chip size, lowering thecost of the memory system. Since the system LSI becomes smaller incircuit scale, it is possible to reduce the time necessary for thedesign verification of the system LSI.

According to another aspect of the memory system of the presentinvention, the common bus is formed on a printed-circuit board formounting the controller and the memory chips. Sharing the memorycontroller among the plurality of memory chips can reduce the number ofsignal lines to be laid on the printed-circuit board, lowering thedesign cost and manufacturing cost of the printed-circuit board.

According to another aspect of the memory system of the presentinvention, the controller and the memory controller are stacked in threedimensions. The common bus is formed as interconnection wiring forconnecting the controller and the memory chips. Sharing the memorycontroller among the plurality of memory chips can reduce the number ofinterconnection wires, thereby allowing an improvement in thereliability of the memory system stacked in three dimension.

BRIEF DESCRIPTION OF THE DRAWINGS

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings in which like parts aredesignated by identical reference numbers, in which:

FIG. 1 is a block diagram showing a memory system having conventionalmemory chips;

FIG. 2 is a system block diagram showing a first embodiment of thepresent invention;

FIG. 3 is a block diagram showing the details of the system LSI of FIG.2;

FIG. 4 is a wiring diagram showing the details of the common bus of FIG.2;

FIG. 5 is a waveform chart showing the interface specifications of thecommon bus of FIG. 2;

FIG. 6 is an explanatory diagram showing the interface classes of thememory system;

FIG. 7 is a timing chart showing read operations of the NOR type flashmemory and the SDRAM in the first embodiment;

FIG. 8 is a timing chart showing a read operation of the NOR type flashmemory and a write operation of the SDRAM in the first embodiment;

FIG. 9 is a timing chart showing write operations of the NOR type flashmemory and the SDRAM in the first embodiment;

FIG. 10 is a timing chart showing read operations of the NAND type flashmemory and the SDRAM in the first embodiment;

FIG. 11 is a timing chart showing a write operation of the NAND typeflash memory and a read operation of the SDRAM in the first embodiment;

FIG. 12 is a wiring diagram showing the details of a common busaccording to a second embodiment of the present invention;

FIG. 13 is a timing chart showing read operations of the NOR type flashmemory and the SDRAM in the second embodiment;

FIG. 14 is a timing chart showing read operations of the NAND type flashmemory and the SDRAM in the second embodiment;

FIG. 15 is a timing chart showing a write operation of the NAND typeflash memory and a read operation of the SDRAM in the second embodiment;

FIG. 16 is a timing chart showing DMA transfer from the NAND type flashmemory to the SDRAM in the second embodiment; and

FIG. 17 is a system block diagram showing a third embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

FIG. 2 shows a first embodiment of the memory system in the presentinvention.

The memory system comprises a system LSI 12 and three clock synchronousmemory chips 14 (an SDRAM 14 a, a NOR type flash memory 14 b, and a NANDtype flash memory 14 c) which are mounted on a printed-circuit board 10.The system LSI 12 and the memory chips 14 a, 14 b, and 14 c areconnected to each other through a common bus 16 formed on theprinted-circuit board 10 and signal lines to be described later.Incidentally, the printed-circuit board 10 contains other electroniccomponents which are not shown, and operates as a main board of, forexample, a portable Internet terminal or the like. That is, theprinted-circuit board 10 operates as a portable system havingpredetermined functions. The memory system is a set of functions of thisportable system that are required for memory operation.

FIG. 3 shows the details of the system LSI 12.

The system LSI includes an MPU 18 (controller) for controlling thememory chips 14 a, 14 b, and 14 c, peripheral cores (IP cores) 20 a, 20b, and 20 c having predetermined functions, and a memory controller 22which is common to the memory chips 14 a, 14 b, and 14 c. The memorycontroller 22 includes an operation memory unit 24, an arbiter 26, aconversion control unit 28, a signal holding unit 30, and aninput/output controlling unit 32.

The operation memory unit 24 stores the operation specifications of thememory chips 14 a, 14 b, and 14 c. For example, when the MPU 18 accessesthe memory chip 14 a (SDRAM) for read operation, the operation memoryunit 24 outputs to the conversion control unit 28 information such asthe order of commands and addresses to be supplied to the SDRAM and thenumber of clocks (latency) from the supply of a command to the output ofdata.

The arbiter 26 adjusts the order of accesses to a plurality of memorychips 14 when the accesses to the memory chips 14 overlap. Specifically,when the MPU 18 instructs read of the memory chip 14 a (SDRAM) and theninstructs, before the completion of the read operation, read of thememory chip 14 b (NOR type flash memory), the arbiter 26 instructs theconversion control unit 28 not to execute the processing on the memorychip 14 b. At the same time, the arbiter 26 instructs the signal holdingunit 30 to hold the signals that are supplied from the MPU 18 regardingthe access to the memory chip 14 b.

The operation memory unit 24 is composed of programmable logics capableof rewriting information stored in themselves. The arbiter 26 iscomposed of programmable logics capable of reconstructing theircircuits. The information of the operation memory unit 24 and thecircuit functions of the arbiter 26 are programmed in accordance withthe memory chips 14 to be connected to the common bus 16. Therefore, thememory controller 22 can be used as a general purpose IP core. Theelements to constitute the programmable logics may be either volatile ornonvolatile.

The conversion control unit 28 controls the input/output controllingunit 32 and the signal holding unit 30 in accordance with theinformation from the operation memory unit 24 and the instruction fromarbiter 26. For example, when the MPU 18 accesses the memory chip 14 a(SDRAM) for read operation, the conversion control unit 28 instructs thesignal holding unit 30 to divide the held address signal into a rowaddress signal and a column address signal for output. It also instructsthat the command signal for instructing the read operation be dividedinto an active command and a read command for output. In the meantime,the conversion control unit 28 instructs an input/output controllingcircuit 32 b on the output timing with which the address signals and thecommand signals are output from the signal holding unit 30.

Based on the information (read latency) from the operation memory unit24, the conversion control unit 28 instructs the input/output circuit 32b (to be described later) in the timing with which it accepts a readdata signal output from the SDRAM 14 a (memory output signal MOUT in thecommon bus 16). In addition, when the MPU 18 is busy, the conversioncontrol unit 28 instructs the signal holding unit 30 to hold theaccepted read data signal temporarily. When the MPU 18 is ready, theread data signal is output through the signal holding unit 30 directlyas a controller input signal CIN. Here, the conversion control unit 28instructs an input/output circuit 32 a (to be described later) on thetiming with which the read data signal is output as the controller inputsignal CIN.

The signal holding unit 30, as mentioned above, temporarily holdscontroller output signals COUT output from the MPU 18 and memory outputsignals MOUT output from the memory chips 14 under the instructions fromthe arbiter 26 and the conversion control unit 28. The signal holdingunit 30 also outputs the held controller output signals COUT and memoryoutput signals MOUT to the input/output circuits 32 b and 32 a,respectively.

The input/output controlling unit 32 has the input/output circuit 32 afor inputting/outputting signals to/from the MPU 18 (system bus) and theinput/output circuit 32 b for inputting/outputting signals to/from thememory chips 14 (common bus 16). The input/output circuit 32 a receivesthe controller output signals COUT that are output from the MPU 18, insynchronization with a timing signal that is output from the conversioncontrol unit 28, and outputs the received signals to the signal holdingunit 30. Besides, the input/output circuit 32 a outputs the memoryoutput signals MOUT that are held in the signal holding unit 30 as thecontroller input signals CIN, in synchronization with a timing signaloutput from the conversion control unit 28.

The input/output circuit 32 b receives the memory output signals MOUTthat are output from the memory chips 14, in synchronization with atiming signal output from the conversion control unit 28, and outputsthe received signals to the signal holding unit 30. The input/outputcircuit 32 b also outputs the controller output signals COUT that areheld in the signal holding unit 30 as memory input signals MINreceivable (recognizable) to the respective memory chips 14, insynchronization with a timing signal output from the conversion controlunit 28.

That is, the conversion control unit 28 controls the operation timingand input/output directions of the input/output circuits 32 a and 32 b.

The controller output signals COUT include address signals, commandsignals, and write data signals output from the MPU 18. The controllerinput signals CIN include read data signals to be supplied from thememory chips 14 to the MPU 18. The address signals output from the MPU18 contain upper address signals for generating the decode signals ofthe memory chips 14 a, 14 b, and 14 c (chip enable signals to bedescribed later).

The memory output signals MOUT include read data signals output from thememory chips 14. The memory input signals MIN include address signals,command signals, and write data signals to be supplied to the memorychips 14. Among the memory output signals MOUT not included in thecommon bus 16 are status signals (busy signals) to be output from theflash memories 14 b and 14 c. Among the memory input signals MIN notincluded in the common bus 16 are the chip enable signals and chipselect signals.

As described above, the memory controller 22 converts the controlleroutput signals COUT output from the processor 18 into the memory inputsignals MIN receivable to the memory chips 14 according to the operationspecifications of the memory chips 14 to operate. The memory chips 14receive the memory input signals MIN through the common bus 16 andperform a read operation, a write operation, or the like. The memorycontroller 22 also receives the memory output signals MOUT output fromthe memory chips 14 through the common bus 16 and converts the receivedsignals into controller input signals ON which are receivable to the MPU18.

FIG. 4 shows the details of the signals for connecting the memorycontroller 22 and the memory chips 14 a, 14 b, and 14 c. In the diagram,the shaded thick arrows and the system clock signal line SCLK areincluded in the common bus 16.

The memory controller 22 (system LSI 12) has a clock terminal CLK and aplurality of status terminals STS0 and STS1 as input terminals, aplurality of chip enable terminals CE0, CE1, CE2, . . . , 4-bit commandterminals COM0-COM3, and 23-bit address terminals ADD0-ADD22 as outputterminals, and 8-bit data input/output terminals DQ0-DQ7 as input/outputterminals.

The SDRAM 14 a has a clock terminal CLK, a chip select terminal /CS,command terminals /RAS, /CAS, and /WE, and address terminals ADD0-ADD13(including bank address terminals) as input terminals, and datainput/output terminals DQ0-DQ7 as input/output terminals. Since theSDRAM 14 a adopts an address multiplex system, the address terminalsADD0-ADD13 are supplied with a row address RA0-RA13 and a column addressCA0-CA8 in succession. The upper two bits (RA12, RA13) of the rowaddress signal are used as bank address signals.

The NOR type flash memory 14 b has a clock terminal CLK, a chip enableterminal /CE, command terminals /WE and /OE, and address terminalsADD0-ADD22 as input terminals, a status terminal STS as an outputterminal, and data input/output terminals DQ0-DQ7 as input/outputterminals.

The NAND type flash memory 14 c has a clock terminal CLK, a chip enableterminal CE, and command terminals CLE, ALE, /RE and /WE as inputterminals, a status terminal STS as an output terminal, and datainput/output terminals I/O0-I/O7 as input/output terminals.

Incidentally, the leading “/”s of terminal names indicate negativelogic. In the following description, signals supplied through terminalswill be designated by the same symbols as those of the terminals, like“clock signal CLK”. Moreover, terminal names and signal names may beabbreviated, like “clock terminal CLK” as “CLK terminal” and “clocksignal CLK” as “CLK signal”.

The CLK terminals of the memory controller 22 and the memory chips 14 a,14 b, and 14 c are supplied with a system clock signal SCLK which isgenerated on the printed-circuit board 10 shown in FIG. 2. The CE0-CE2terminals of the memory controller 22 are connected to the /CS terminalof the SDRAM 14 a, the /CE terminal of the flash memory 14 b, and the CEterminal of the flash memory 14 c, respectively.

The memory controller 22 outputs signals of negative logic from the CE0and CE1 terminals and a signal of positive logic from the CE2 terminalbased on the information from the operation memory unit 24 shown in FIG.3. The COM0-COM3 terminals of the memory controller 22 are connected tothe command terminals of the SDRAM 14 a and the flash memories 14 b, 14c. When the SDRAM 14 a is accessed, the COM3 terminal will not be used.Similarly, when the flash memory 14 b is accessed, neither the COM2terminal nor the COM3 terminal will be used.

The address terminals ADD0-ADD22 of the memory controller 22 areconnected to the address terminals of the SDRAM 14 a and the flashmemory 14 b. The flash memory 14 c (NAND type) has no address terminal,and thus is not connected with the address terminals ADD0-ADD22.

The data input/output terminals DQ0-DQ7 of the memory controller 22 areconnected to the data input/output terminals DQ0-DQ7, I/O0-I/O7 of theSDRAM 14 a and the flash memories 14 b, 14 c. The STS0 and STS1terminals of the memory controller 22 are connected to the STS terminalsof the flash memories 14 b and 14 c, respectively.

As described above, the command signal lines, address signal lines, anddata input/output signal lines for connecting the memory controller 22to the SDRAM 14 and the flash memories 14 b, 14 c are shared to form thecommon bus. Therefore, the number of wires to be formed on theprinted-circuit board 10 is reduced as compared to heretofore. Thisdecreases, for example, the number of wiring layers on theprinted-circuit board 10, lowering the design cost and fabrication costof the printed-circuit board 10.

Since the number of terminals of the memory controller 22 is reduced ascompared to heretofore, the system LSI 12 is prevented from growing insize depending on the number of terminals.

The system LSI 12 decreases in circuit scale, reducing the timenecessary for design verification.

FIG. 5 shows the interface specifications of the common bus 16.

The input signals to be input to the common bus 16 must be settled asetup time tIS before a rising edge of the SCLK signal and maintained atthe settled level (VIH or VIL) until a hold time tIH (input timingspecification). The output signals to be output from the common bus 16must be settled in output an access time tAC after a rising edge of theSCLK signal and maintained until a hold time tOH from another risingedge of the SCLK signal (output timing specification).

In this embodiment, the common bus 16 has a clock cycle tCLK of 10 ns.Here, the setup time tIS, the hold time tIH, the access time tAC, andthe hold time tOH are defined as 1.5 ns, 0.8 ns, 5.4 ns, and 1.8 ns,respectively. Given that the clock cycle tCLK is 10 ns, the setup timetOS of an output signal with respect to the rising edge of the SCLKsignal is 4.6 ns.

The memory controller 22 and the memory chips 14 a, 14 b, and 14 c haveonly to input/output signals to/from the common bus 16 in accordancewith the foregoing interface specifications. That is, simply definingthe four times, i.e., the setup time tIS, the hold time tIH, the setuptime tOS, and the hold time tOH allows transmission of commands,addresses, and data between the memory controller 22 and the memorychips 14 a, 14 b, and 14 c through the common bus 16. Data can also betransmitted among the memory chips 14 a, 14 b, and 14 c through thecommon bus 16. These interface specifications are characterized by thatthe input timing specification and the output timing specificationremain the same for the memory chips 14 a, 14 b, and 14 c. That is, theinterface specifications are independent of the operation specificationsinherent to the memory chips 14 a, 14 b, and 14 c.

When clock synchronous memory chips are developed anew, the memory chipscan be connected to the memory controller 22 by designing input/outputcircuits in accordance with the interface specifications shown in FIG.5. That is, the memory chips can be attached to the system LSIexternally without developing a new memory controller 22.

Note that the clock cycle tCLK is not limited to this example, but maybe determined in accordance with the operating frequencies of the MPUcore 18 and the memory chips 14 a, 14 b, and 14 c. Here, some changesmay be made to the setup times and hold times of the input and outputsignals according to the clock cycle tCLK.

On the printed-circuit board 10 shown in FIG. 2, the rules of the commonbus 16, such as wiring length, are determined so as to meet theinterface specifications shown in FIG. 5. When these rules are followed,signals that are supplied from the system LSI 12 to the common bus 16 inaccordance with the requirements of the input timing specification areoutput to the memory chip 14 a (or 14 b, 14 c) within the requirementsof the output timing specification. Similarly, signals that are suppliedfrom the memory chip 14 a (or 14 b, 14 c) to the common bus 16 inaccordance with the requirements of the input timing specification areoutput to the system LSI 12 within the requirements of the output timingspecification.

FIG. 6 shows the interface classes of the memory system.

In the diagram, the first class is an interface level in which therising and falling characteristics of signals are defined. In thisclass, the input/output characteristics of signals are determined asVLTTL, SSTL, or the like. The second class is a timing level in whichthe input/output timing of signals is defined with respect to the clocksignal. The third class is an operation level (command level) to bedefined depending on the operation specifications of the respectivememory chips.

In the present embodiment, the memory controller 22 and the memory chips14 a, 14 b, and 14 c are interfaced at the second class (timing level).Accordingly, the command signals, address signals, and data input/outputsignals can be shared as the common bus 16 among the plurality of typesof memory chips 14 a, 14 b, and 14 c. The conventional memory systemshown in FIG. 1 was interfaced at the third level (operation level). Forthis reason, the bus wiring was conventionally required for each memorychip.

FIG. 7 shows an example where the system LSI accesses the NOR type flashmemory 14 b and the SDRAM 14 a in succession to perform read operations.The “system bus” in the diagram shows signals to be transmitted betweenthe MPU 18 and the memory controller 22. The “common bus 16” showssignals to be transmitted between the memory controller 22 and the SDRAM14 a (or the flash memory 14 b).

The MPU 18 outputs a read command RD and an address (14 b) insynchronization with the initial SCLK signal (0th) (FIG. 7( a)). Thememory controller 22 decodes an upper address out of the address (14 b)supplied to the system bus, to detect that the MPU 18 is requestingaccess to the flash memory 14 b.

The conversion control unit 28 shown in FIG. 3 receives, for example,read operation specifications (1)-(3) of the flash memory 14 b from theoperation memory unit 24.

-   (1) A read operation is started upon the reception of a read command    RD and a read address ADD.-   (2) A read latency is “8”. That is, first data is output at the    eighth clock after the supply of the read command RD.-   (3) Read data has a burst length of “4”.

The memory controller 22 activates the CE1 signal (/CE signal) insynchronization with the rising edge of the next SCLK signal (first),and outputs a read command RD and a read address ADD to the flash memory14 b (FIG. 7( b)). Here, the memory controller 22 outputs the CE1signal, the read command RD, and the read address ADD in accordance withthe interface specifications for input signals shown in FIG. 5. Theflash memory 14 b receives the read command RD and the read address ADDthrough the common bus 16 (FIG. 7( c)), and performs a read operation.Here, the read command RD, the read address ADD, and the CE1 signal thatthe flash memory 14 b receives from the common bus 16 meet the interfacespecifications for output signals shown in FIG. 5.

The MPU 18 outputs a read command RD and an address (14 a) insynchronization with the first SCLK signal (FIG. 7( d)). The memorycontroller 22 decodes an upper address out of the address (14 a)supplied to the system bus, to detect that the MPU 18 is requestingaccess to the SDRAM 14 a.

The conversion control unit 28 shown in FIG. 3 receives, for example,read operation specifications (1)-(4) of the SDRAM 14 a from theoperation memory unit 24.

-   (1) A read operation is started upon the reception of an active    command ACT and a row address signal RA. The row address signal RA    is the 14 upper bits of an address, including bank address signals    BA0 and BA1.-   (2) A read command RD and a column address signal CA become    receivable one or more clocks after the supply of the active command    ACT. The column address signal CA is the nine lower bits of the    address.-   (3) A read latency is “2”. That is, first data is output at the    second clock after the supply of the read command RD.-   (4) Read data has a burst length of “4”.

The memory controller 22 activates the CE0 signal (/CS signal) insynchronization with the rising edge of the next SCLK signal (second),and outputs an active command ACT and a row address RA to the SDRAM 14 a(FIG. 7( e)). Here, the memory controller 22 outputs the CE0 signal, theactive command ACT, and the row address RA in accordance with theinterface specifications for input signals shown in FIG. 5. The SDRAM 14a receives the active command ACT and the row address RA through thecommon bus 16 (FIG. 7( f)), and operates such internal circuits as a rowdecoder and a sense amplifier. Here, the active command ACT, the rowaddress RA, and the CE0 signal that the SDRAM 14 a receives from thecommon bus 16 meet the interface specifications for output signals shownin FIG. 5. Incidentally, the internal circuits of the SDRAM 14 a operateeven after the inactivation of the /CS signal.

Based on the information from the operation memory unit 24, theconversion control unit 28 shown in FIG. 3 determines that the readcommand RD to the SDRAM 14 a cannot be supplied until after the outputof data from the flash memory 14 b. Therefore, the memory controller 22keeps the CE1 signal activated (FIG. 7( g)).

The flash memory 14 b outputs read data signals D0-D3 to the common bus16 in succession (FIG. 7( h)). Here, the flash memory 14 b outputs theread data signals D0-D3 in accordance with the interface specificationsfor input signals shown in FIG. 5. The memory controller 22 receives theread data signals D0-D3 with the input/output circuit 32 b of FIG. 3 insuccession, and temporarily stores the received data into the signalholding unit 30. Here, the read data signals D0-D3 that the memorycontroller 22 receives from the common bus 16 meet the interfacespecifications for output signals shown in FIG. 5. The conversioncontrol unit 28 controls the signal holding unit 30 and the input/outputcircuit 32 a so that the held data is successively output to the systembus in synchronization with the 10th and subsequent SCLK signals (FIG.7( i)). Then, the read operation of the flash memory 14 b is completed.

Next, the memory controller 22 activates the CE0 signal insynchronization with the 13th SCLK signal, and outputs a read command RDand a column address signal CA (FIG. 7( j)). The SDRAM 14 a outputs readdata signals D0-D3 to the common bus 16 in succession two clocks afterthe supply of the read command RD (FIG. 7( k)). The memory controller 22receives the read data signals D0-D3 with the input/output circuit 32 bin succession, and temporarily stores the received data into the signalholding unit 30. Here, the read data signals D0-D3 that the memorycontroller 22 receives from the common bus 16 meet the interfacespecifications for output signals shown in FIG. 5. The conversioncontrol unit 28 controls the signal holding unit 30 and the input/outputcircuit 32 a so that the held data is successively output to the systembus in synchronization with the 16th and subsequent SCLK signals (FIG.7( l)). Then, the read operation of the SDRAM 14 a is completed.

FIG. 8 shows an example where the system LSI accesses the NOR type flashmemory 14 b and the SDRAM 14 a in succession to perform a read operationof the flash memory 14 b and a write operation of the SDRAM 14 a.Detailed description will be omitted of the same operations as those ofFIG. 7.

The MPU 18 outputs a read command RD and an address (14 b) insynchronization with the initial SCLK signal (0th) (FIG. 8( a)). Thememory controller 22 activates the CE1 signal (/CE signal) insynchronization with the rising edge of the next SCLK signal (first),and outputs a read command RD and a read address ADD to the flash memory14 b (FIG. 8( b)). The flash memory 14 b receives the read command RDand the read address ADD through the common bus 16 (FIG. 8( c)), andperforms a read operation.

The MPU 18 outputs a write command WR and a write address (14 a) insynchronization with the first SCLK signal (FIG. 8( d)). The MPU 18successively outputs write data signals D0-D3 in synchronization withthe first to fourth SCLK signals. These commands, addresses, and dataare temporarily stored into the signal holding unit 30. The memorycontroller 22 decodes an upper address out of the address (14 a)supplied to the system bus, to detect that the MPU 18 is requestingaccess to the SDRAM 14 a. The conversion control unit 28 shown in FIG. 3receives, for example, write operation specifications (1)-(4) of theSDRAM 14 a from the operation memory unit 24.

-   (1) A write operation is started upon the reception of an active    command ACT and a row address signal RA. The row address signal RA    is the 14 upper bits of an address, including the bank address    signals BA0 and BA1.-   (2) A write command WR and a column address signal CA become    receivable one or more clocks after the supply of the active command    ACT. The column address signal CA is the nine lower bits of the    address.-   (3) A write latency is “0”. That is, write data signals are    successively output along with the write command WR.-   (4) Write data has a burst length of “4”.

The memory controller 22 activates the CE0 signal (/CS signal) insynchronization with the rising edge of the second SCLK signal, andoutputs an active command ACT and a row address RA to the SDRAM 14 a(FIG. 8( e)). The SDRAM 14 a receives the active command ACT and the rowaddress RA through the common bus 16 (FIG. 8( f)), and operates suchinternal circuits as a row decoder and a sense amplifier.

Based on the information from the operation memory unit 24, theconversion control unit 28 shown in FIG. 3 determines that the writecommand WR to the SDRAM 14 a can be supplied before the output of datafrom the flash memory 14 b. Accordingly, the controller 22 reactivatesthe CE0 signal (FIG. 8( g)), and outputs a write command WR and a columnaddress signal CA to the common bus 16 in synchronization with thefourth SCLK signal (FIG. 8( h)). The memory controller 22 successivelyoutputs the write data signals D0-D3 to the common bus 16 insynchronization with the fourth to seventh SCLK signals (FIG. 8( i)).The SDRAM 14 a accepts the write data signals D0-D3 in succession andperforms a write operation (FIG. 8( j)).

Subsequently, as in FIG. 7, the flash memory 14 b outputs read datasignals D0-D3 to the common bus 16 in succession at and after the eighthclock from the supply of the read command RD, thereby performing a readoperation (FIG. 8( k)).

FIG. 9 shows an example where the system LSI accesses the SDRAM 14 a andthe NOR type flash memory 14 b in succession to perform a writeoperation of the SDRAM 14 a and a write operation of the flash memory 14b. Detailed description will be omitted of the same operations as thoseof FIGS. 7 and 8.

The MPU 18 outputs a write command WR and an address (14 a) insynchronization with the initial SCLK signal (0th) (FIG. 9( a)). The MPU18 successively outputs write data signals D0-D3 in synchronization withthe zeroth to third SCLK signals. The memory controller 22 decodes anupper address out of the address (14 a) supplied to the system bus, todetect that the MPU 18 is requesting access to the SDRAM 14 a.

The memory controller 22 activates the CE0 signal (/CS signal) insynchronization with the rising edge of the first SCLK signal, andoutputs an active command ACT and a row address RA to the SDRAM 14 a(FIG. 9( b)). The SDRAM 14 a receives the active command ACT and the rowaddress RA (FIG. 9( c)), and operates such internal circuits as a rowdecoder and a sense amplifier.

Since the system bus is not supplied with a next command, the controller22 reactivates the CE0 signal in synchronization with the third SCLKsignal (FIG. 9( d)), and outputs a write command WR and a column addresssignal CA to the common bus 16 (FIG. 9( e)). The memory controller 22successively outputs the write data signals D0-D3 to the common bus 16in synchronization with the third to sixth SCLK signals (FIG. 9( f)).The SDRAM 14 a accepts the write data signals D0-D3 in succession andperforms a write operation (FIG. 9( g)).

The MPU 18 outputs a write command WR and an address (14 b) insynchronization with the fourth SCLK signal (FIG. 9( h)). The MPU 18successively outputs write data signals D0-D3 in synchronization withthe fourth to seventh SCLK signals. The memory controller 22 decodes anupper address out of the address (14 b) supplied to the system bus, todetect that the MPU 18 is requesting access to the flash memory 14 b.

The conversion control unit 28 shown in FIG. 3 receives, for example,write operation specifications (1)-(5) of the flash memory 14 b from theoperation memory unit 24.

-   (1) A write operation is started upon the reception of a write    command WR and a write address ADD.-   (2) A write latency is “0”. That is, write data signals are    successively output along with the write command WR.-   (3) Write data has a burst length of “4”.-   (4) After the write data signals are received, the STS signal is    kept at a high level until the completion of the data write (BUSY    period).-   (5) No command, address, nor data can be input during the BUSY    period.

The conversion control unit 28 receives from the arbiter 26 theinformation indicating that the SDRAM 14 a is in operation. Theconversion control unit 28 makes the signal holding unit 30 hold thecommand, address, and data for the flash memory 14 b which are suppliedfrom the MPU 18. The signal holding unit 30 is controlled by theconversion control unit 28 so as to output the held write command WR,write address ADD, and write data signals D0-D3 in synchronization withthe seventh and subsequent SCLK signals at which the operation of theSDRAM 14 a is completed. Then, the write operation of the flash memory14 b is performed (FIG. 9( j)).

The flash memory 14 b activates the STS signal while performing thewrite operation, thereby notifying the memory controller 22 of the busystate (FIG. 9( k)). The memory controller 22 monitors the STS signal insynchronization with the SCLK signal. The memory controller 22 detectsthe STS signal turning to a low level, and then informs the MPU 18 thatthe flash memory 14 b is in a ready state. The MPU 18 is informed of theready state, for example, via the signal line of a BUSY signal formed onthe system bus.

To verify that the flash memory 14 b is written with correct data, theMPU 18 instructs a read operation under the address identical to thewrite address (FIG. 9( l)). Then, a read operation of the flash memory14 b is performed at the same timing as in FIG. 7 (FIG. 9( m)).

FIG. 10 shows an example where the system LSI accesses the NAND typeflash memory 14 c and the SDRAM 14 a in succession to perform readoperations. Detailed description will be omitted of the same operationsas those of FIG. 7.

The MPU 18 outputs a read command RD and an address (14 c) insynchronization with the initial SCLK signal (0th) (FIG. 10( a)). Thememory controller 22 decodes an upper address out of the address (14 c)supplied to the system bus, to detect that the MPU 18 is requestingaccess to the flash memory 14 c.

The conversion control unit 28 shown in FIG. 3 receives, for example,read operation specifications (1)-(5) of the flash memory 14 c from theoperation memory unit 24.

-   (1) A read operation is started when a command latching signal CL    and a read command RD are received at the command terminals    COM0-COM3 and the data input/output terminals DQ0-DQ7, respectively,    in synchronization with a clock signal.-   (2) An address latching signal AL and read address signals ADD    (start address) are received in synchronization with the second to    fourth clock signals.-   (3) A read data length is set in a mode register or the like (“4” in    this example).-   (4) After the read address is received, the STS signal is kept at a    high level until read data signals become ready for output (BUSY    period).-   (5) No command, address, nor data can be input during the BUSY    period.

The MPU 18 outputs a read command RD and an address (14 a) insynchronization with the first SCLK signal (FIG. 10( b)). The memorycontroller 22 decodes an upper address out of the address (14 a)supplied to the system bus, to detect that the MPU 18 is requestingaccess to the SDRAM 14 a. The read command RD and the address (14 a) aretemporarily held in the signal holding unit 30.

The memory controller 22 activates the CE2 signal (CE signal) insynchronization with the rising edge of the first SCLK signal, andoutputs a command latching signal CL and a read command RD to the flashmemory 14 c (FIG. 10( c)). The memory controller 22 successively outputsan address latching signal AL and address signals ADD (start address) insynchronization with the second to fourth SCLK signals (FIG. 10( d)).

The flash memory 14 c receives the command latching signal CL, the readcommand RD, the address latching signal AL, and the address signals ADDthrough the common bus 16 in succession (FIG. 10( e)), and performs aread operation. Incidentally, the read operation (internal operation ofthe flash memory 14 c) is performed even after the inactivation of theCE signal.

The flash memory 14 c activates the STS signal until read data signalsbecome ready for output, thereby notifying the memory controller 22 ofthe busy state (FIG. 10( f)).

Based on the information from the operation memory unit 24, theconversion control unit 28 determines that the read operation of theSDRAM 14 a can be performed before the reception of the read datasignals from the flash memory 14 c. The memory controller 22 activatesthe CE0 signal (/CS signal) in synchronization with the rising edge ofthe fifth SCLK signal, and outputs an active command ACT and a rowaddress RA to the SDRAM 14 a (FIG. 10( g)). The SDRAM 14 a receives theactive command ACT and the row address RA (FIG. 10( h)), and operatessuch internal circuits as a row decoder and a sense amplifier.

The memory controller 22 reactivates the CE0 signal in synchronizationwith the seventh SCLK signal (FIG. 10( i)), and outputs a read commandRD and a column address signal CA (FIG. 10( j)). The SDRAM 14 a outputsread data signals D0-D3 to the common bus 16 in succession two clocksafter the supply of the read command RD (FIG. 10( k)). The conversioncontrol unit 28 controls the signal holding unit 30 and the input/outputcircuit 32 a so that the read data signals D0-D3 from the SDRAM 14 athat are held in the signal holding unit 30 are successively output tothe system bus in synchronization with the ninth and subsequent SCLKsignals (FIG. 10( l)). Then, the read operation of the SDRAM 14 a iscompleted.

Next, the memory controller 22 monitors the STS signal insynchronization with the SCLK signal. The memory controller 22 detectsthe STS signal turning to a low level, and then activates the CE2 signaland outputs a read command RD (FIG. 10( m)). The flash memory 14 coutputs read data signals D0-D3 in succession two clocks after thereception of the read command RD (FIG. 10( n)).

The read data signals D0-D3 are successively output to the system bus insynchronization with the 16th and subsequent SCLK signals (FIG. 10( o)).Then, the read operation of the flash memory 14 c is completed.

FIG. 11 shows an example where the system LSI accesses the NAND typeflash memory 14 c and the SDRAM 14 a in succession to perform a writeoperation of the flash memory 14 c and a read operation of the SDRAM 14a. Detailed description will be omitted of the same operations as thoseof FIG. 7.

The MPU 18 outputs a write command WR and an address (14 c) insynchronization with the initial SCLK signal (0th) (FIG. 11( a)). Inaddition, the MPU 18 successively outputs write data signals DQ0-DQn insynchronization with the zeroth and subsequent SCLK signals (FIG. 11(b)). The memory controller 22 decodes an upper address out of theaddress (14 c) supplied to the system bus, to detect that the MPU 18 isrequesting access to the flash memory 14 c.

The conversion control unit 28 shown in FIG. 3 receives, for example,write operation specifications (1)-(7) of the flash memory 14 c from theoperation memory unit 24.

-   (1) A write operation is started when a command latching signal CL    and a write command WR are received at the command terminals    COM0-COM3 and the data input/output terminals DQ0-DQ7, respectively,    in synchronization with a clock signal.-   (2) An address latching signal AL and write address signals ADD    (start address) are received in synchronization with the second to    fourth clock signals.-   (3) In synchronization with the fifth and subsequent clock signals,    a data latching signal DL and write data signals D0-Dn are received    at the command terminals COM-COM3 and the data input/output    terminals DQ0-DQ7, respectively.-   (4) A read data length is set in a mode register or the like of the    flash memory 14 c (“n+1” in this example).-   (5) In synchronization with the clock signal subsequent to the    reception of the write data signal Dn, a command latching signal CL    and a program start signal PST are received at the command terminals    COM-COM3 and the data input/output terminals DQ0-DQ7, respectively.-   (6) After the program start signal PST is received, the STS signal    is kept at a high level until the completion of the data write (BUSY    period).-   (7) No command, address, nor data can be input during the BUSY    period.

The memory controller 22 activates the CE2 signal (CE signal) insynchronization with the rising edge of the next SCLK signal (first),and outputs a command latching signal CL and a write command WR to theflash memory 14 c (FIG. 11( c)). The memory controller 22 successivelyoutputs an address latching signal AL and address signals ADD insynchronization with the second to fourth SCLK signals (FIG. 11( d)).The memory controller 22 successively outputs a data latched signal DLand write data signals DQ0-DQn in synchronization with the fifth andsubsequent SCLK signals (FIG. 11( e)). The CE2 signal is kept at thehigh level until the output of a program start signal PST (FIG. 11( f)).

The flash memory 14 c receives the command latching signal CL, the writecommand WR, the address latching signal AL, the address signals ADD, thedata latching signal DL, and the write data signals DQ0-DQn through thecommon bus 16 in succession (FIG. 11( g)), and performs a readoperation.

The flash memory 14 c activates the STS signal until the completion ofthe write operation, notifying the memory controller 22 of the busystate (FIG. 11( h)).

The MPU 18 outputs a read command RD and an address (14 a) to the flashmemory 14 c in synchronization with the SCLK signal subsequent to theoutput of the write data signal DQn (FIG. 11( i)). The memory controller22 decodes an upper address out of the address (14 a) supplied to thesystem bus, to detect that the MPU 18 is requesting access to the SDRAM14 a. The read command RD and the address (14 a) are temporarily held inthe signal holding unit 30.

Based on the information from the operation memory unit 24, theconversion control unit 28 determines that the read operation of theSDRAM 14 a can be performed after the output of the read data signals tothe flash memory 14 c.

The memory controller 22 activates the CE0 signal (/CS signal) insynchronization with the rising edge of the SCLK signal subsequent tothe output of the program start signal PST, and outputs an activecommand ACT and a row address RA to the SDRAM 14 a (FIG. 11( j)). Then,as in FIG. 10, a read command RD and a column address signal CA areoutput from the memory controller 22 (FIG. 11( k)) so that the readoperation of the SDRAM 14 a is performed.

As has been described, in the present embodiment, the memory controller22 converts controller output signals COUT output by the MPU 18 intomemory input signals MIN receivable to the memory chips 14, according tothe operation specifications of the respective memory chips 14. Thisallows the single memory controller 22 to access the plurality of typesof memory chips 14. Since the plurality of memory chips 14 can beconnected to the memory controller 22 through the common bus 16, thesignal lines can be minimized in number. Besides, the memory controller22 can be reduced in circuit scale.

The input timing specifications on the memory input signals MIN and thememory output signals MOUT which the memory controller 22 and the memorychips 14 respectively input to the common bus 16 is set identicalirrespective of which of the memory chips 14 is to operate. Similarly,the output timing specifications on the memory output signals MOUT andthe memory input signals MIN to be output to the memory controller 22and the memory chips 14 through the common bus 16 is set identicalirrespective of which of the memory chips 14 is to operate. Therefore,the memory controller 22 can make reliable access to the plurality oftypes of memory chips 14 having different operation specifications bysimply adjusting the order of output of the memory input signals MIN andthe order of acceptance of the memory output signals MOUT according tothe command specifications of the memory chips 14.

The setup time tOS and the hold time tOH of the output timingspecification are set longer than the setup time tIS and the hold timetIH of the input timing specification. Accordingly, the memorycontroller 22 and the individual memory chips 14 can surely receive thememory output signals MOUT and the memory input signals MIN through thecommon bus 16, respectively.

The input/output controlling unit 32, or the interface with the memorychips 14, outputs the memory input signals MIN and receives the memoryoutput signals MOUT by operating under the timing according to theoperation specifications of the respective memory chips 14.Consequently, it is possible to operate the memory chips 14 reliablywithout using complicated control circuits.

The controller output signals COUT and the memory output signals MOUTreceived at the input/output controlling unit 32 are temporarily held bythe signal holding part 30. Therefore, the signals can be output to thememory chips 14 according to the operation specifications of therespective memory chips 14.

The signal holding unit 30 can hold controller output signals COUT untilthe common bus 16 becomes available. This allows the MPU 18 to accessother devices, such as peripheral circuits, or the peripheral cores 20a, 20 b, and 20 c independent of the operation wait for the memory chips14. Since the MPU 18 is prevented from executing useless cycles, theentire system can be improved in operating efficiency.

The operation memory unit 24 is composed of programmable logics that arecapable of rewriting information stored in themselves. In addition, thearbiter 26 is composed of programmable logics that can reconstruct theirrespective circuit functions. On this account, the control timing of thememory controller 22 can be modified easily by programming the operationmemory unit 24 and the arbiter 26 depending on the memory chips 14 to beconnected to the memory controller 22. As a result, the memorycontroller 22 can be used as a controller that is common to a number oftypes of memory chips 14.

When access is requested of a plurality of memory chips 14, the order inwhich the memory chips 14 operates is adjusted by the arbiter 26 and thesignal holding unit 30. This allows the single memory controller 22 tooperate the plurality of types of memory chips 14 with efficiency. Thememory system can be improved in data transmission rate.

The memory controller 22 can handle the plurality of types of memorychips 14 by itself, and thus can be made smaller in circuit scale. As aresult, the system LSI 12 for mounting the memory controller 22 on canbe reduced in chip size, lowering the cost of the memory system. Sincethe system LSI 12 decrease in circuit scale, it is possible to reducethe time necessary for the design verification of the system LSI 12.

The memory controller 22 is shared among the plurality of memory chips14 to be mounted on the printed-circuit board 10. This can reduce thenumber of signal lines to be laid on the printed-circuit board 10,lowering the design cost and fabrication cost of the printed-circuitboard 10.

FIG. 12 shows a second embodiment of the memory system in the presentinvention. The same circuits and signals as those described in the firstembodiment will be designated by identical reference numbers or symbols.Detailed description thereof will be omitted here.

In this embodiment, a memory controller 34 has command terminalsCOM0-COM2, COM3-COM4, and COM5-COM8 corresponding to memory chips 14 a,14 b, and 14 c, respectively. That is, the shaded thick arrows and thesystem clock signal line SCLK in the diagram are included in a commonbus 16. In addition, the memory controller 34 receives a control signalDMA which is output from a not-shown MPU 18. The DMA signal is activated(high level) when the MPU 18 instructs the memory controller 34 of DMA(Direct Memory Access) transfer. The other configuration is almostidentical to that of the first embodiment described above.

FIG. 13 shows an example where the system LSI accesses the NOR typeflash memory 14 b and the SDRAM 14 a in succession to perform readoperations. FIG. 13 shows operations corresponding to FIG. 7 of thefirst embodiment. Detailed description will be omitted of the sameoperations as those of FIG. 7.

Initially, the read operation of the flash memory 14 b is performed asin FIG. 7. The memory controller 34 outputs a read command RD and acolumn address CA to the SDRAM 14 a in synchronization with the 11thSCLK signal. This timing is two clocks earlier than in the firstembodiment. Here, the CE0 signal and the CE1 signal are activated at thesame time, while no signal collision occurs on the common bus 16. Then,the read data signals D0-D3 from the SDRAM 14 a are output insynchronization with the 14th to 17th SCLK signals. The rest of thetiming is the same as in FIG. 7.

Since the read data signals from the flash memory 14 b and the read datasignals D0-D3 from the SDRAM 14 a are output continuously, the memorysystem improves in data transfer efficiency as compared to the firstembodiment.

FIG. 14 shows an example where the system LSI accesses the NAND typeflash memory 14 c and the SDRAM 14 a in succession to perform readoperations. FIG. 14 shows operations corresponding to FIG. 10 of thefirst embodiment. Detailed description will be omitted of the sameoperations as those of FIG. 10.

Initially, the read operations of the flash memory 14 c and the SDRAM 14a are started as in FIG. 10. The memory controller 34 outputs a readcommand RD to the flash memory 14 c in synchronization with the 11thSCLK signal. This timing is two clocks earlier than in the firstembodiment. Here, the CE0 signal and the CE2 signal are activated at thesame time, while no signal collision occurs on the common bus 16. Then,the read data signals D0-D3 from the flash memory 14 c are output insynchronization with the 14th to 17th SCLK signals. The rest of thetiming is the same as in FIG. 10. Even in this example, the memorysystem improves in data transfer efficiency as compared to the firstembodiment.

FIG. 15 shows an example where the system LSI accesses the NAND typeflash memory 14 c and the SDRAM 14 a in succession to perform a writeoperation of the flash memory 14 c and a read operation of the SDRAM 14a. FIG. 15 shows operations corresponding to FIG. 11 of the firstembodiment. Detailed description will be omitted of the same operationsas those of FIG. 11.

Initially, the write operation of the flash memory 14 c is started as inFIG. 11. The memory controller 34 outputs an active command ACT and aread command RD to the SDRAM 14 a while outputting write data signals tothe flash memory 14 c. This timing is four clocks earlier than in thefirst embodiment. Here, the CE0 signal and the CE2 signal are activatedat the same time, while no signal collision occurs on the common bus 16.The rest of the timing is the same as in FIG. 11. Even in this example,the memory system improves in data transfer efficiency as compared tothe first embodiment.

FIG. 16 shows an example of DMA transfer from the flash memory 14 c tothe SDRAM 14 a. The basic operations of the flash memory 14 c and theSDRAM 14 a are the same as in FIGS. 10 and 11 described above.Therefore, detailed description of the operations will be omitted here.

For DMA transfer, the MPU 18 turns the DMA signal to a high level whenoutputting the read command RD to the flash memory 14 c and the writecommand WR to the SDRAM 14 a (FIG. 16( a)). On account of DMA transfer,the MPU 18 outputs no write data signal. That is, only a write addressAD and the write command WR are supplied to the SDRAM. The memorycontroller 34 activates the CE2 signal (CE signal) in synchronizationwith the rising edge of the first SCLK signal, and outputs the readcommand RD and read addresses ADD to the flash memory 14 c (FIG. 16(b)). The flash memory 14 c receives the read command RD and the readaddresses ADD (FIG. 16( c)), and performs a read operation.

The memory controller 34 outputs an active command ACT and a row addressRA to the SDRAM 14 a in synchronization with the 10th SCLK signal (FIG.16( d)). The memory controller 34 outputs a read command RD to the flashmemory 14 c in synchronization with the 11th SCLK signal (FIG. 16( e)).

The flash memory 14 c outputs read data signals D0-D3 in succession twoclocks after the supply of the read command RD (the 13th SCLK signal)(FIG. 16( f)). In synchronization with this 13th SCLK signal, the memorycontroller 34 outputs a write command WR and a column address CA to theSDRAM 14 a (FIG. 16( g)). As a result, the read data output from theflash memory 14 c are written to the SDRAM 14 a via the common bus 16.That is, a DMA transfer is performed. During the DMA transfer, thememory controller 34 accepts none of the read data signals D0-D3.

As described above, this embodiment can offer the same effects as thoseobtained from the first embodiment described above. Moreover, in thisembodiment, the signal lines of the command signals are separated fromthe common bus 16 and laid with respect to each memory chip. On thisaccount, the memory controller 34 can activate a plurality of chipenable signals CE0-CE2 at a time. For example, a read command can besupplied to one memory chip while another memory chip isinputting/outputting data signals to the common bus 16. As a result, thememory system can be improved in data transmission rate.

The signal lines of the address signal ADD0-ADD22 and the data signalDQ0-DQ7 are included in the common bus 16, while the signal lines of thecommand signals are separated from the common bus 16 and laid for eachmemory chip. This facilitates DMA transfer between the memory chips.During the DMA transfer, the MPU 18 can access other peripheral circuitsor IP cores. Consequently, the system improves in performance.

FIG. 17 shows a third embodiment of the memory system in the presentinvention.

In this embodiment, a system LSI 36, an SDRAM 38 a, and flash memories38 b and 38 c are stacked in three dimensions and molded in a singlepackage (not shown). A common bus 16 is formed as interconnection wiringfor connecting the individual chips via through holes that are formed inthe peripheries of the respective chips. The common bus 16 has the sameinterface specifications as those of FIG. 5.

The system LSI 36, the SDRAM 38 a, and the flash memories 38 b and 38 chave the same circuit configurations as those of the system LSI 12, theSDRAM 14 a, and the flash memories 14 b and 14 c of the firstembodiment. That is, the system LSI 36 includes the memory controller22. The memory chips 38 a, 38 b, and 38 c are clock synchronoussemiconductor memories.

This embodiment can offer the same effects as those obtained from thefirst embodiment described above. Moreover, in this embodiment, thecommon bus 16 is formed as the interconnection wiring for connecting theindividual chips 36, 38 a, 38 b, and 38 c via the through holes formedin the peripheries of the respective chips. This makes it possible toform a memory system with a minimum mounting area. Sharing the memorycontroller among the plurality of memory chips can reduce the number ofinterconnection wires, thereby allowing an improvement in thereliability of the memory system stacked in three dimensions.

The first and second embodiments described above have dealt with thecases where the memory chips 14 a, 14 b, and 14 c each have datainput/output terminals of 8 bits. However, the present invention is notlimited to such embodiments. For example, the data input/outputterminals may be of 16 bits. Memory chips of 8 bits and 16 bits may beused together. In this case, the common bus has data input/output signallines of 16 bits.

The first embodiment described above has dealt with the case where thememory system comprises the clock synchronous SDRAM 14 a, the NOR typeflash memory 14 b, and the NAND type flash memory 14 c. However, thepresent invention is not limited to such an embodiment. For example, thememory system may include a clock synchronous SSRAM (Synchronous SRAM).

The invention is not limited to the above embodiments and variousmodifications may be made without departing from the spirit and thescope of the invention. Any improvement may be made in part or all ofthe components.

What is claimed is:
 1. A common memory controller capable of controllingchips including a respective memory among different types of memoriesoperating with different specifications, comprising: a first interfaceportion that receives a control output signal to access the chips from acontroller, and outputs a control input signal to the controller; asecond interface portion that outputs a memory input signal through acommon bus terminal portion coupled to the chips and receives a memoryoutput signal through the common bus terminal portion; an operationmemory portion that outputs a first control signal includingspecifications of the memories; an arbiter that outputs a second controlsignal and a third control signal; a conversion control portion thatconverts the control output signal into the memory input signal inresponse to at least one of the first control signal and the secondcontrol signal, and converts the memory output signal respectively intothe control input signal; and a signal holder that holds the controloutput signals in response to at least one of the third control signaland a fourth control signal from the conversion control portion, whereinthe memory input signal includes a first memory input signalcorresponding to a first memory and a second memory input signalcorresponding to a second memory, type of which is different from thetype of the first memory.
 2. The common memory controller according toclaim 1, wherein the controller outputs a first control output signalcorresponding to a first operation of the first memory and a secondcontrol output signal corresponding to a second operation of the secondmemory, and the arbiter outputs the second control signal to theconversion control portion when the second control output signal isoutput from the controller before the first operation is completed. 3.The common memory controller according to claim 1, wherein the secondinterface portion is coupled to at least one of the chips via a commonbus.
 4. The common memory controller according to claim 1, wherein thecontroller outputs a first control output signal corresponding to afirst operation of the first memory and a second control output signalcorresponding to a second operation of the second memory, and thearbiter outputs the third control signal to the signal holder when thesecond control output signal is output from the controller before thefirst operation is completed.
 5. The common memory controller accordingto claim 1, wherein the conversion control portion outputs the fourthcontrol signal to the signal holder when the controller is at a busystate.
 6. The common memory controller according to claim 1, wherein thechips are stacked in three dimensions.
 7. The common memory controlleraccording to claim 6, wherein each of the chips includes at least onehole in a peripheral area and the hole of one chip among the chips iscoupled to the hole of the other chip among the chips withinterconnection wiring.
 8. The common memory controller according toclaim 1, wherein the first memory and the second memory include asynchronous DRAM or a synchronous SRAM.
 9. The common memory controlleraccording to claim 6, wherein the memory input signal further includes achip select signal.
 10. The common memory controller according to claim6, wherein the memory input signal further includes a direct memoryaccess control signal.
 11. The common memory controller according toclaim 1, further comprising: a memory unit for storing the specificationof memories, and wherein the memory unit is programmable.
 12. The commonmemory controller according to claim 1, wherein the specification ofmemories includes a memory type, a command sequence, an address sequenceor a latency information.
 13. The common memory controller according toclaim 1, wherein the second interface unit controls an output timing ofthe memory input signal in response to the specification of each of thememories.
 14. The common memory controller according to claim 1, whereinthe memory input signal includes an address, a command and data.
 15. Thecommon memory controller according to claim 1, wherein a number ofterminals of common bus terminal portion is larger than a number ofterminals of each of the memories to which the memory controller isconnected.
 16. The common memory controller according to claim 1,wherein the common bus terminal portion includes control terminals,address terminal, data terminals and a clock terminal.
 17. A memorysystem comprising: a plurality of chips including a respective memoryamong a plurality of different types of memories operating withdifferent specifications and in synchronization with a clock signal; acontroller that outputs controller output signals to access the memorychips; a memory controller that controls access of the plurality ofchips; and a common bus coupled to the chips and the memory controllerto transmit memory input signals and memory output signals correspondingto a control signal, wherein the command specifications for the chipsare different so that the memory input signals of the chips aredifferent, wherein the memory controller includes a first interface unitthat receives control output signals from the controller to access thechips and outputs control input signals to the controller; a secondinterface unit that outputs the memory input signals to the memory viathe common bus and receives the memory output signals from the memoryvia the common bus; an operation memory unit that stores an operationspecification of the respective memory; an arbiter that outputs a firstinstruction signal and a second instruction signal; a conversion controlunit that converts, based on the specification stored in the operationmemory unit and the first instruction signal from the arbiter, thecontrol output signals into the memory input signals and the memoryoutput signals into the control input signals; and a signal holding unitthat temporarily holds the controller output signals in response to thesecond instruction signal for operating one of the chips during thecontroller operation of another one of the chips.
 18. The memory systemaccording to claim 17, wherein the memory output signals received by thememory controller and the memory input signals received by the chipsboth through the common bus have the same input timing specificationirrespective of which of the chips is to operate; and the memory inputsignals output from the memory controller and the memory output signalsoutput from the chips both through the common bus have the same outputtiming specification irrespective of which of the chips is to operate.19. The memory system according to claim 17, wherein the arbiter adjuststhe order of accesses to the chips when access requests to the chipsoverlap.
 20. The memory system according to claim 17, wherein the memorycontroller and the controller are mounted on a single chip.